Rename the misleading variable name in setEnabled()

The meaning of the variable "isEnabled" and the logic part is very misleading in setEnabled() function, which implemented by  many profile classes in settingslib.

The return value of setConnectionPolicy() called in setEnabled() is true if connectionPolicy is set, false on error. The variable's name maybe better called as "isSuccessful" instead of "isEnabled" for correct meaning of this value.

Bug: 254188027
Test: build pass
Change-Id: If1026495e70311096c483177b6d00ab714c87227